The present invention relates to a system for detecting the presence of formation gas in a stream of drilling fluid flowing through a wellbore during drilling of the wellbore. In the search for hydrocarbon fluids present in earth formation layers it is important to detect the inflow of gas from the earth formation into the wellbore during drilling, at an early stage. If the gas is at high pressure early detection is crucial to ensure proper well control and to forego inadvertent wellbore conditions. Furthermore, the inflow of gas in the wellbore fluid provides valuable information on the various earth formation layers crossed by the wellbore. Thus, gas inflow may be indicative for hitting a hydrocarbon rich prospect or for an emerging hazardous blowout. Gas species which are most common in earth formation layers are methane (CH4), carbon dioxide (CO2) and nitrogen (N2). Also H2S may be encountered during wellbore drilling.
These gas species may occur either as free-gas bubbles or as gas dissolved-in-liquid.
Until now it has been difficult to detect the presence of earth formation gases during wellbore drilling in a timely and accurate manner.